1. Field of the Invention
The present invention relates in general to wireless communication systems and methods and RF power amplifiers and methods.
2. Description of the Prior Art and Related Information
The two primary goals of RF power amplifier design are linearity over the range of power operation and efficiency. Linearity is simply the ability to amplify without distortion while efficiency is the ability to convert DC to RF energy with minimal wasted power and heat generation. Both these requirements are critical for modern wireless communication systems but it is increasingly difficult to provide both. This is due primarily to the bandwidth requirements of modern wireless communication systems which are placing increasing demands on amplifier linearity. As a practical matter the only way to provide the desired linearity has been to employ very large amplifiers operating in a low efficiency point of their operating range where they are more linear.
More specifically, linearization of RF power amplifiers is inherently difficult to achieve as RF power amplifiers use a large number of non-linear devices which become more and more nonlinear at increasingly higher output power levels. In practice, high power RF devices will generate substantial unwanted InterModulation Distortion (IMD) products which appear as spurious signals at the output of the RF power amplifier. Depending on the input signal type at the input of the power amplifier these unwanted signals may appear as spectral regrowth around the base of the wideband signal, e.g., in a CDMA (Code Division Multiple Access) system, or as additional carriers if more than one signal carrier is applied at the input of the amplifier. In general, wireless service providers around the world are subject to many governmental rules and regulations, which mandate very strict bandwidth usage. Spectrum constraints, as well as increasing output power levels drive performance requirements for RF power amplifiers. To meet these requirements large amplifiers operated in a highly linear but relatively inefficient point in their operating range have been used.
In addition to limiting IMDs in response to regulatory and signal quality requirements, RF power amplifiers employed in modern wireless communication systems must be efficient, i.e., must achieve good DC to RF conversion efficiencies, to avoid unnecessary power dissipation and heat. This requirement is increasing in importance driven by shrinking volumes of deployment facilities (e.g. cellular base stations), as well as reduction in heat exchanger size (or smaller air conditioning units), and reduction in operating noise levels due to cooling fans, as well as other factors. This need for efficiency clearly runs counter to the above noted need for large amplifiers operated at a linear but inefficient operating point to achieve desired linearity.
Although there are many different approaches to achieving higher linearity and good efficiency in RF power amplifiers, feed forward amplifiers provide a common approach. In feed forward RF power amplifiers an error amplifier is employed to amplify only IMD products which are then combined with the main amplifier output to cancel the main amplifier IMDs. FIG. 1 illustrates a conventional feed forward amplifier design having a main amplifier 1 and an error amplifier 2. The basic elements also include delays 3, 4 in the main and error path, respectively, and main to error path couplers 5, 6, 7 and 8. Additional elements not shown are also typically present in a conventional feed forward architecture as is well known to those skilled in the art. The delays, couplers and error amplifier are designed to inject out of phase IMDs from the error path into the main amplifier output at coupler 8 to substantially eliminate the IMDs in the main amplifier path. Typically the main amplifier size in a feed forward system is chosen to be big enough to handle all or most signal peaks and the error amplifier size is relatively small as schematically illustrated in FIG. 1. For example, the error amplifier in conventional feed forward power amplifiers is typically about one ninth the size of the main amplifier. The average error amplifier power dissipation is thus quite small. Nonetheless, the main amplifier is large and its efficiency is quite low and thus the overall feed forward amplifier efficiency is quite low. An obvious way to improve main amplifier efficiency is to employ a smaller amplifier and drive it harder, however, this introduces unacceptably large IMDs for modern high bandwidth applications.
Therefore, a need presently exists for an RF power amplifier design which provides both high efficiency and minimal distortion in broad bandwidth RF applications.
The present invention provides a feed forward RF power amplifier design which provides both high efficiency and minimal distortion in broad bandwidth RF applications.
In a first aspect the present invention provides a feed forward amplifier comprising an RF input for receiving an RF signal and a main amplifier receiving and amplifying the RF signal, wherein the main amplifier is biased in a first bias class of operation. The feed forward amplifier also comprises a main amplifier output sampling coupler, a first delay coupled to the RF input and providing a delayed RF signal and a carrier cancellation combiner coupling the delayed RF signal to the sampled output from the main amplifier. The feed forward amplifier further comprises an error amplifier receiving and amplifying the output of the carrier cancellation combiner. The error amplifier is biased in a second bias class of operation with higher linearity than the first bias class. A second delay is coupled to the output of the main amplifier and an error injection coupler combines the output from the error amplifier and the delayed main amplifier output from the second delay so as to cancel distortion introduced by the main amplifier. An RF output is coupled to the error injection coupler output and provides an amplified RF output.
In a preferred embodiment the ratio of main amplifier to error amplifier size is from 2 to 1 or from 1 to 2. The first bias class of operation is preferably class C or class AB2 and the second bias class of operation is preferably class A or class AB1. The main amplifier may comprise one or more semiconductor amplifier devices, for example, plural LDMOS amplifier devices, and the device bias current in the first bias class of operation is preferably between 0 and 0.17 percent of device saturation current or between 1.25 and 2.50 percent of device saturation current. The error amplifier may also comprise one or more semiconductor amplifier devices, for example, plural LDMOS amplifier devices, and the device bias current in the second bias class of operation is preferably between 3.33 and 10.00 percent of device saturation current or between 10.00 and 25.00 percent of device saturation current. The feed forward amplifier may also further comprise a pre-distortion circuit coupled to the input of the main amplifier and a controller for controlling the operation of the pre-distortion circuit to minimize distortion at the amplifier RF output.
In another aspect the present invention provides a feed forward amplifier, comprising an RF input for receiving an RF input signal, the RF input signal having an average operating amplitude range and intermittent signal peaks in a peak range exceeding the average operating range. For example, the RF input signal may comprise a spread spectrum signal, such as a CDMA signal or a WCDMA signal, having randomly occurring signal peaks which comprise the peak signal range. The feed forward amplifier includes a main amplifier receiving and amplifying the RF input signal, the main amplifier having a first transfer characteristic over its range of operation. The first transfer characteristic has a substantially linear portion corresponding to the average operating amplitude range of the RF input signal and a nonlinear portion corresponding to the RF input signal peak range. The feed forward amplifier also includes a main amplifier output signal sampler, an error path delay circuit coupled to the RF input and providing a delayed RF input signal, and a first cancellation combiner coupling the delayed RF input signal to the sampled output from the main amplifier. The feed forward amplifier further includes an error amplifier for amplifying the output of the first cancellation combiner. The error amplifier has a second transfer characteristic over its range of operation, the second transfer characteristic having a linear portion corresponding to substantially all of the average and peak operating amplitude range of the RF input signal. The feed forward amplifier further includes a main path delay circuit coupled to the output of the main amplifier, a second cancellation combiner combining the output from the error amplifier and the output of the main path delay circuit so as to cancel distortion introduced by the main amplifier, and an RF output coupled to the second cancellation combiner and providing an amplified RF output.
In one specific implementation the range of operation of the error amplifier may be about 30 dB. The average power range of operation of the error amplifier may be about 10 dB. The power vs gain transfer characteristic of the error amplifier is preferably linear to less than 0.5 dB of gain through about 25 dB or more of the 30 dB operating range. Alternatively, the power vs gain transfer characteristic of the error amplifier may be linear up to about xe2x88x924 to xe2x88x925 dB from peak device power. The range of operation of the main amplifier may be from about xe2x88x9220 dB from peak power to peak power and the range of operation of the error amplifier may be from about xe2x88x9230 dB from peak power to peak power. The feed forward amplifier may further comprise a pre-distortion circuit coupled to the input of the main amplifier and a controller for controlling the operation of the pre-distortion circuit to minimize distortion at the amplifier output. The feed forward amplifier may further comprise a pilot signal generator providing a pilot signal to the input of the main amplifier and a pilot signal detector coupled to the amplifier output and the controller.
In another aspect the present invention provides a method for amplifying a broad bandwidth RF input signal. The method comprises receiving an RF input signal having an average operating amplitude range and intermittent signal peaks in a peak signal range exceeding the average operating range. For example, the RF input signal may comprise a spread spectrum signal having randomly occurring signal peaks which comprise the peak signal range. The method comprises amplifying the RF input signal employing a main amplifier having a first transfer characteristic over its range of operation, the first transfer characteristic having a substantially linear portion corresponding to the average operating amplitude range of the RF input signal and a nonlinear portion corresponding to the RF input signal peak range. The method further comprises sampling the main amplifier output, delaying the RF input signal and providing a delayed RF input signal, and coupling the delayed RF input signal to the sampled output from the main amplifier so as to provide a distortion component of the sampled output from the main amplifier. The method further comprises amplifying the distortion component employing an error amplifier having a second transfer characteristic over its range of operation, the second transfer characteristic having a linear portion corresponding to substantially all of the average and peak operating amplitude range of the RF input. The method further comprises delaying the output of the main amplifier, combining the amplified distortion component and the delayed output of the main amplifier so as to cancel distortion introduced by the main amplifier and providing an amplified RF output.
In a preferred embodiment, the range of operation of the error amplifier may be about 30 dB. The average power range of operation of the error amplifier may be about 10 dB. The power vs gain transfer characteristic of the error amplifier is preferably linear to less than 0.5 dB of gain through about 25 dB or more of the 30 dB operating range. Alternatively, the power vs gain transfer characteristic of the error amplifier may be linear up to about xe2x88x924 to xe2x88x925 dB from peak device power. The range of operation of the main amplifier may be from about xe2x88x9220 dB from peak power to peak power and the range of operation of the error amplifier may be from about xe2x88x9230 dB from peak power to peak power. The method may further comprise pre-distorting the RF input signal prior to amplifying by the main amplifier.
Further aspects of the invention will be appreciated from the following detailed description of the invention.